FIG. 1 shows various anatomical structures associated with the head of a horse. Among these, the airway structures, and in particular the larynx and pharynx, are susceptible to various disorders which affect the horse's health and its ability to perform normally. The larynx is innervated by the recurrent laryngeal nerves (RLN) which contain motor fibers that innervate both the abductor/opener and adductor/closer muscles of the arytenoid cartilages and their associated vocal folds. The soft palate normally fits closely around the ventral border of the epiglottis.
Airway impairment produces two symptoms, abnormal sounds and/or poor performance. Abnormal sounds are detected by the human ear (riders, judges) and can be quantitate by time-frequency spectrogram. Horses produce inspiratory sounds characterized by three frequency bands centered at approximately 0.3, 1.6, and 3.8 k Hz; See Derksen F J et al., Spectrum Analysis Of Respiratory Sounds In Exercising Horses With Experimentally Induced Laryngeal Hemiplegia Or Dorsal Displacement Of The Soft Palate, Am J Vet Res. 2001 May; 62(5):659-64, incorporated herein by reference. Respiratory sounds of horses have been recorded using a radiostethoscope such as that disclosed by Attenburrow et al., Resonant Frequency of the Lateral Ventrical and Saccule and Whistling, Equine Exercise Physiology, pp 27-32, and in U.S. Pat. No. 4,218,584 to Attenburrow, both of which describe a stethoscope for detecting and recording data from a horse while the horse is walking, trotting, cantering, jumping, and galloping. A transducer such as a microphone is attached to the animal's skin adjacent to the windpipe. The electrical output from the transducer is transferred to a radio transmitter mounted on the animal or its harness. The radio transmitter can transmit signals a distance from the horse to allow for monitoring the horse's breathing from a distance. U.S. Pat. No. 6,228,037 describes a method and apparatus for recording and analysis of respiratory sounds in exercising horse, and U.S. Pat. No. 6,659,960 describes a method and system for continuous monitoring and diagnosis of body sounds, which discloses a portable unit for recording the upper airway respiratory sounds of an exercising horse to determine whether the horse suffers from an upper airway obstruction condition.
Some horses exhibit a disorder of the upper airway known as dorsal displacement of the soft palate (DDSP). Pulmonary ventilation at rest is adequate, but during exercise the cross sectional area of the horse pharynx is reduced by collapse during exhalation. This results in significant airflow reduction which is generally associated with an abnormal upper respiratory noise at exercise. In horses used for competition, the decreased volume of airflow interferes with performance and may impair the horse's ability to compete. Although conventional methods of treatment have been useful in some horses, they are less than ideal since they have only modest success rates and significant complications.
The specific pathophysiology of DDSP is that during exercise horses normally interlock their soft palate and the epiglottis to form a direct open airway from the nasal cavity to the trachea. But in some horses, the soft palate displaces dorsally during exercise, the free end of the palate then lies in the airway and causes a major obstruction to expired air. The exact cause of DDSP is not known, however, it is believed to be caused by either direct mechanical displacement by posterior movements of the tongue, or weakness in the muscles of the soft palate or those that raise the epiglottis or the entire larynx.
Several muscles are related to movement of the larynx and pharynx, but the specific role of each is not well understood. US Patent Publication 2007123950 described a twelve muscle model of the pharyngeal and laryngeal airway and showed that neuromuscular stimulation of just one or two of the muscles sufficed to move the hyoid bone. Two airway muscles could be coordinated synergistically to protect the pharyngeal and laryngeal airway by a distance reduction of the hyoid complex and larynx relative to the chin. Specifically, stimulating the thyrohyoid muscle reduced the distance of the hyoid complex relative to the larynx. And geniohyoid muscle stimulation in combination with at least one other muscle also generated the synergy of simultaneous reduction of distance of the hyoid complex and the larynx relative to the chin. Multiple intramuscular electrodes were placed in two or more upper airway muscles and controlled by an implanted stimulation device.
Functional electrical stimulation (“FES”) refers to the application of stimulation devices to nerves and muscles to treat medical disorders. Application of FES to paralyzed laryngeal muscles was introduced into human clinical otolaryngology in 1977 by Zealear D L, Dedo H H, Control Of Paralyzed Axial Muscles By Electrical Stimulation, Acta Otolaryngol (Stockholm) 1977, 83:514-27, incorporated herein by reference. The most successful FES system to date is the cardiac pacer which has become a routine part of cardiac disease therapy: Lynch, Cardiovascular Implants, in Implants, Lynch ed., Van Nostrand Rheinhold, New York 1982, incorporated herein by reference. However, there are a variety of other FES systems, of which the most heavily researched are FES systems to restore locomotion to paraplegics and arm motion to quadriplegics: Peckham, IEEE Trans. Biomed. Eng. 1991, 28: 530, incorporated herein by reference. Other motor control devices restore bladder control to paraplegics and diaphragm function to high quadriplegics: Erlandson, Scand. J. Urol. Nephrol. 44 Suppl: 31, 1978; Glenn, Ann. Surg. 183: 566, 1976, incorporated herein by reference. There also are FES devices designed to rehabilitate the sensory deficits, such as the cochlear implant: Hambrecht, Ann. Otol. Rhinol. Laryngol. 88: 729, 1979, incorporated herein by reference. To date such systems have not been developed for horses which present different clinical conditions from humans.
An FES device for the equine applications must be effective and also must conform to the rules of the governing bodies that oversee equine sports. In thoroughbred and standard breed racing, this requires that the device must not give the horse an unfair advantage. In addition, it cannot allow tampering with the horse's performance. Specifically, as wagering is an integral part of the sport, there cannot be a way of adjusting the device to manipulate the horse's performance.